1. Field of the Invention
The present invention relates to a photovoltaic power generation system that increases alternatives to allow connection of solar cell elements or solar cell element groups (solar cell modules) used for composing a solar cell string installed on a structure such as a roof and that enables accurate tracking of the maximum output point of a solar cell string.
2. Description of the Related Art
FIG. 21 is a block diagram showing a related art photovoltaic power generation system 17. As show in FIG. 21, there is conventionally proposed a photovoltaic power generation system 17 made by connecting a plurality of solar cell strings 11a, 11b in parallel and composed so as to enable acquirement of a maximum output electric power from the solar cell strings 11a, 11b. This photovoltaic power generation system 17 comprises solar cell strings 11a, 11b, each of which is made of a plurality of solar cell elements connected to each other that convert solar energy to electric energy, a connection box 13 that connects backflow prevention diodes 19 for preventing an output current of one of the solar cell strings from flowing back to another solar cell string and serves as connecting means for collecting output voltages from the plural solar cell strings 11a, 11b, and a power conditioner 14 constituted by an electric power converting device for converting DC electric power outputted from the respective solar cell strings 11a, 11b to AC electric power whose voltage/current phase is synchronized with that of a commercial electric power system 16 and a protecting device for detecting any trouble of the commercial electric power system. AC electric power outputted from the power conditioner 14 is supplied to an AC load 15 such as a light or a motor.
In general, in the case of obtaining necessary electric power by using a solar cell string, solar cell strings 11a, 11b are composed of solar cell elements or solar cell modules 18 connected to each other in series, in parallel or in series-parallel, and composed so as to enable acquirement of a voltage and a current that a power conditioner 14 executing electric power conversion can efficiently convert.
However, since the amount of generated electric power of a solar cell string changes depending on an insolation condition and an environment, a point referred to as the maximum output point that a current and voltage of a solar cell string can be taken out most efficiently changes at all times. Then, for the purpose of obtaining the maximum output electric power of the solar cell strings, the power conditioner 14 is controlled so that an operation point of the power conditioner 14 tracks varying maximum output points of the solar cell strings.
However, since the power conditioner 14 controls output electric power of the solar cell strings as a result of collection in the connection box 13, it is impossible to control output electric power of the respective solar cell strings 11a, 11b. Therefore, in order to match output voltages between the solar cell strings, there is a need to equalize the numbers of solar cell elements between the solar cell strings. For example, in the case of the plural solar cell strings 11a, 11b connected in parallel that have different numbers of solar cell elements, the maximum output points of the respective solar cell strings locate on different points, with the result that the efficiency of power generation is lowered in the mass when the power conditioner 14 is operated at a certain output voltage. This results in decrease in conversion efficiency when the composition number of the solar cell modules 18 is arbitrarily set, so that it is impossible to serve a need to, for example, lay the solar cell modules 18 on the entire roof of a house and keep high conversion efficiency.
In recent years, on a house or the like with solar cell modules mounted in which emphasis is laid on appearance, harmony with its environment and individuality, dummy modules which do not generate electric power are placed together with the solar cell module 18 to fill in space, or solar cell modules which are prevented from generating electric power are placed in a vacant part. Moreover, since the unit of expansion of the solar cell modules 18 is a string unit, there arises a case that a difference is made between a necessary capacity of electric power generation and an actual installable capacity, and it is impossible to cause a remaining installable area smaller than that of an installation region of one solar cell string to contribute to electric power generation, with the result that such a problem arises that it is impossible to satisfy a required capacity of electric power generation.
Accordingly, a photovoltaic power generation system that in the case of strings connected in parallel which have different numbers of solar cell elements, voltage regulating means is connected to a string whose number of solar cell elements is smaller so that output voltages from the plural solar cell strings match is disclosed in Japanese Unexamined Patent Publication JP-A 2001-312319.
However, in the above method, because it is required to manually select a boosting voltage ratio by using a contact selection switch disposed to the voltage regulating means at the time of installation of the photovoltaic power generation system, the man-hours at the time of installation increase, and moreover, in the case of mistakenly setting the contact selection switch, an output corresponding to the installation number of the solar cell modules 18 cannot be obtained, and such a problem arises that the photovoltaic power generation system does not work properly.
Moreover, a boosting ratio is fixed at a set value when the boosting voltage ratio is manually set. Therefore, when the installation directions of the solar cell strings are different from each other, a difference is made in operation points for obtaining the maximum outputs of the solar cell strings 11a, 11b because of differences in conditions of insolation to the respective solar cell modules and module temperature conditions. However, since the boosting voltage ratio is fixed, such a problem arises that it is impossible to obtain the true maximum output electric power.
Furthermore, it is necessary to check and control whether setting is properly done at the time of building, and collection, recording and control of the result and work for making a builder do it thoroughly are required.